Scilab Function
Last update : Dec 1997

cdfchi - cumulative distribution function chi-square distribution

### Calling Sequence

  `[P,Q]=cdfchi("PQ",X,Df)`
  `[X]=cdfchi("X",Df,P,Q);`
  `[Df]=cdfchi("Df",P,Q,X)`

### Parameters

- `P,Q,Xn,Df`: four real vectors of the same size.
- `P,Q (Q=1-P)` : The integral from 0 to X of the chi-square distribution. Input range: [0, 1].
- `X`: Upper limit of integration of the non-central chi-square distribution. Input range: [0, +infinity). Search range: [0,1E300]
- `Df`: Degrees of freedom of the chi-square distribution. Input range: (0, +infinity). Search range: [ 1E-300, 1E300]

### Description

Calculates any one parameter of the chi-square distribution given values for the others.

Formula 26.4.19 of Abramowitz and Stegun, Handbook of Mathematical Functions (1966) is used to reduce the chi-square distribution to the incomplete distribution.

Computation of other parameters involve a seach for a value that produces the desired value of P. The search relies on the monotinicity of P with the other parameter.

From DCDFLIB: Library of Fortran Routines for Cumulative Distribution Functions, Inverses, and Other Parameters (February, 1994) Barry W. Brown, James Lovato and Kathy Russell. The University of Texas.